bnx2x: small fix in stats handling

Use a private variable to fold rx_dropped value, instead of shared
destination buffer, as it might break SNMP applications.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: Eilon Greenstein <eilong@broadcom.com>
CC: Dmitry Kravkov <dmitry@broadcom.com>
Acked-By: Eilon Greenstein <eilong@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>